Scorpio,
The desert can get HOT! I would talk to someone who could advise on what R rating you would need for your situation. I'm fortunate to live in a mild climate but the summer's do get HOT and the shed will get the afternoon sun for sure. I think I'll be fine with the heat as long as I can keep the room exhausted frequently. My intake is 4" but I was told to run 6" ducting, fan and exhaust to keep that air moving which is really important. If the heat gets out of hand, I'll have to resort to a small a/c unit. Since this is my first venture into the "indoor world", I'm not expecting great things right away but am hoping for the best.